A polycrystalline silicon etching solution and its application

Abstract

This invention discloses a polycrystalline silicon etching solution and its application. The etching solution comprises aromatic carboxylic acids, silane coupling agents, aliphatic carboxylic acids, phosphoric acid, and water. The polycrystalline silicon etching solution of this invention can maintain a high etching rate even at high silica concentrations, and produces a low surface roughness after etching, making it suitable for etching deep-hole structures.

Description

Technical Field

[0001] This invention belongs to the field of electronic chemicals technology, specifically relating to a polycrystalline silicon etching solution and its application. Background Technology

[0002] In flash memory chip technology, 3D NAND technology vertically stacks multiple layers of data storage cells, accommodating more storage cells in a smaller space. Its process has progressed from 128 layers to stacked structures of more than 300 layers.

[0003] In 3D NAND memory, polysilicon material layers need to be filled in the channels through the stacked structure, and wet etching is used in the deep holes left by dry etching to obtain a flat surface and a uniform structure.

[0004] Traditional silicon etching solutions often use a nitric acid-hydrofluoric acid system, but its disadvantages include a rough surface after etching, placing a heavy burden on subsequent surface treatment processes. Soda ash etching, while exhibiting strong crystal orientation, also cannot guarantee uniform etching. An alkaline system of ammonia-hydrogen peroxide can also be used for polycrystalline silicon etching, resulting in lower surface roughness. However, when applied to deep-hole structures, severe silica accumulation at the bottom inhibits etching, leading to excessive differences in etching depth between upper and lower layers.

[0005] In contrast, high-temperature (>165℃) phosphoric acid etching of polycrystalline silicon has advantages such as moderate etching rate, fast mass transfer and isotropic etching. It can also be combined with additives such as silane coupling agents to weaken the influence of silicic acid on the polycrystalline silicon surface.

[0006] To address the above issues, a composite additive needs to be added to phosphoric acid to formulate a polycrystalline silicon etching solution suitable for deep hole etching. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides a polycrystalline silicon etching solution and its application. The polycrystalline silicon etching solution is suitable for etching deep-hole structures, and the etching rate decreases by no more than 5% at higher silicon contents.

[0008] To achieve the above objectives, the present invention provides a polycrystalline silicon etching solution comprising the following raw materials by mass percentage: 1.0-1.5% aromatic carboxylic acid, 1.5-2.0% silane coupling agent, 0.5-1.0% tetrazolium compound, 1.2-1.8% fatty alcohol compound, 90.0% phosphoric acid, and the balance being deionized water.

[0009] Preferably, the aromatic carboxylic acid is any one of benzoic acid, salicylic acid, p-hydroxybenzoic acid, o-methylbenzoic acid, 3,5-dimethylbenzoic acid, p-aminobenzoic acid, and m-aminobenzoic acid.

[0010] Preferably, the silane coupling agent is one or more of methoxytrimethylsilane, ethoxytrimethylsilane, methyltrimethoxysilane, methyltriethoxysilane, ethyltriethoxysilane, aminopropyltrimethoxysilane, aminopropyltriethoxysilane, and γ-aminoethylaminopropyltrimethoxysilane.

[0011] Preferably, the tetrazolium compound is any one of tetrazolium, 1-methyl-1H-tetrazole, 5-methyltetrazole, 5-aminotetrazole, and 5-amino-1-methyltetrazole.

[0012] Preferably, the fatty alcohol compound is any one of n-heptanol, 2-heptanol, 3-heptanol, 4-heptanol, cycloheptanol, n-octanol, 2-octanol, 3-octanol, 4-octanol, isooctol, cyclooctanol, and cyclooctanol.

[0013] Preferably, the phosphoric acid is pure phosphoric acid.

[0014] The present invention also provides an application of a polycrystalline silicon etching solution in deep-hole polycrystalline silicon etching.

[0015] Preferably, the aspect ratio of the deep-hole polycrystalline silicon is 200-300.

[0016] Preferably, the etching temperature is 165-175°C.

[0017] The beneficial effects of this invention are as follows: 1. Polycrystalline silicon is partially oxidized in high-temperature, high-concentration phosphoric acid. Oxygen vacancies exist in the oxide layer. The carboxyl groups in aromatic carboxylic acids can combine with oxygen vacancies to form relatively stable Si-OC bonds, forming a protective layer on the surface of polycrystalline silicon, thereby preventing the silica precipitated from the reaction from sticking back onto the polycrystalline silicon surface.

[0018] 2. Part of the silane coupling agent binds to the silicic acid precipitated in the solution, preventing silicic acid agglomeration and improving its diffusion performance in deep-pore structures; another part binds to silanol groups on the polycrystalline silicon surface, preventing the reaction products from inhibiting etching and ensuring the consistency of etching rates between upper and lower layers in deep-pore structures.

[0019] 3. During the etching process, a small number of bubbles will appear on the polycrystalline silicon surface, which is not conducive to the uniformity of etching. The present invention adds alcohol compounds to the etching solution to increase the solubility of bubbles and organic additives in the etching solution, so that the bubbles can be quickly removed from the polycrystalline silicon surface and the smoothness of the surface after etching can be improved.

[0020] 4. Azole compounds can stabilize silane additives and reaction byproducts, prevent particle precipitation, and reduce the number of residual particles on the polycrystalline silicon surface after etching.

[0021] 5. The polycrystalline silicon etching solution prepared by this invention exhibits an initial etching rate greater than 4 A / min for polycrystalline silicon wafers. When the added silicon content is 500 ppm, the etching rate decreases by no more than 5% compared to the initial etching rate; the roughness Ra value after etching does not exceed 0.3 nm. When etching polycrystalline silicon structural layers with deep holes (aspect ratio of 200-300), it can etch complete and uniform structures, and the difference in etching width between upper and lower layers does not exceed 10%. Attached Figure Description

[0022] Figure 1 This is a cross-sectional schematic diagram of the polycrystalline silicon structure sheet in Example 14.

[0023] Figure 2 The image shows the top of the deep-hole structure sheet after etching with the polycrystalline silicon etching solution prepared in Example 1.

[0024] Figure 3 The image shows the top of the deep-hole structure sheet after etching with the polycrystalline silicon etchant prepared in Comparative Example 1. Detailed Implementation

[0025] The technical solution of the present invention will be further explained and described below with reference to the accompanying drawings and specific embodiments. It is worth noting that the following embodiments are only preferred embodiments of the present invention and should not be construed as limiting the present invention. The scope of protection of the present invention should be determined by the contents of the claims. Modifications and substitutions made by those skilled in the art to the technical solution of the present invention without creative effort all fall within the scope of protection of the present invention.

[0026] Examples 1-13, Comparative Examples 1-4 Aromatic carboxylic acids, azole compounds, and alcohols were added sequentially to phosphoric acid and heated and stirred until dissolved. Then, a silane coupling agent was added at 80°C to react fully with the phosphoric acid to obtain a stable polycrystalline silicon etching solution. The specific formula is shown in Table 1. Table 1 Raw materials and formulation of polysilicon etching solution

[0027] Note: The polysilicon etching solution described in the table above contains 1.2% aromatic carboxylic acid, 1.8% silane coupling agent, 1.5% alcohol compound, 0.6% azole compound, 90% phosphoric acid, and the balance is deionized water.

[0028] Example 14 Etching Experiment (1) Etching the wafer: The polycrystalline silicon film deposited on the silicon semiconductor wafer has a thickness of 250 Å. Before etching, the wafer is cut into strips of 1.5 cm * 3 cm. It is first cleaned with a hydrofluoric acid solution with a volume ratio of 200:1 for 30 s to remove the oxide layer, and then placed in the polycrystalline silicon etching solution prepared in the above examples and comparative examples for etching. The etching temperature is 170 ± 0.5 °C and the etching time is 300 s. (2) Calculation of etching rate: The thickness of silicon oxide and silicon nitride films before and after etching is detected by elliptic polarization spectrometer. The difference between the initial thickness and the thickness after a certain time is divided by the etching time to obtain the etching rate. (3) Silicon content window detection: The polycrystalline silicon etching solution prepared in the above examples and comparative examples has an initial silicon content of 0 ppm. Etching is performed, and then tetraethyl silicate is added to it to prepare a polycrystalline silicon etching solution with a silicon content of 500 ppm. The etching rate of the polycrystalline silicon wafer is tested to characterize the silicon content window. (4) Roughness detection: The etched polycrystalline silicon wafers were cleaned sequentially with deionized water, SC-1 cleaning solution and deionized water, and then roughness was detected by atomic force microscope (AFM). The smaller the Ra value, the lower the roughness. (5) Structural layer etching test: The polycrystalline silicon etching solution prepared using the above examples and comparative examples was used to test the deep hole structural layer ( Figure 1 Etching was performed under the same conditions as (1), and the etching rate was measured in the same way as (2). The etching endpoint was 10 nm etched on one side of the top circular hole. The diameter of the deep hole structure layer was 50 nm and the depth was 15 μm. After etching, the width of the top and bottom was measured by a transmission electron microscope (TEM) to characterize the etching effect.

[0029] Table 2. Etching rate and attenuation ratio of polysilicon wafers with varying silicon content per wafer

[0030] Note: The roughness of the polysilicon wafer before etching is Ra=0.65nm.

[0031] Table 3. Etching amount and difference at the top and bottom of the deep hole in the deep hole structure layer.

[0032] Comparative Example 1 shows the etching of polycrystalline silicon by pure phosphoric acid. The etching rate at a silicon concentration of 500 ppm decreased by 61.42% compared to 0 ppm. At the same time, the silicic acid has a strong back-adhesion effect, resulting in a large difference in the amount of upper layer etching in the deep hole structure.

[0033] Depend on Figure 2 and Figure 3 It can be seen that the roughness after etching in Comparative Example 1 is significantly greater than that in Example 1, which corresponds to the Ra value measured by AFM.

[0034] Comparative Example 2 added benzoic acid to Comparative Example 1. Under its surface protection, the etching rate of polycrystalline silicon with high silicon content decreased to 13.39%.

[0035] Comparative Example 3 added methyltrimethoxysilane to Comparative Example 1, which combined some free silica, thereby reducing the difference in etching amount between the upper and lower layers in the deep hole structure to 16.55%.

[0036] Comparative Example 4 added isopropanol to Comparative Example 1, which enhanced the solubility of the solution for bubbles, resulting in a lower surface roughness of the polycrystalline silicon after etching.

[0037] Comparative Example 5 added 5-aminotetrazole to Comparative Example 1, which can prevent the silicon oxide particles generated by silica agglomeration from adhering to the polycrystalline silicon surface and can also reduce the roughness after etching.

[0038] Examples 1-13 added aromatic carboxylic acids, silane coupling agents, alcohols, and tetrazolium compounds at preferred concentrations, achieving an etching rate decay of less than 5% and a roughness Ra value of less than 0.3 nm for polycrystalline silicon monoliths with a silicon content of 500 ppm, and an etching amount difference of no more than 10% between upper and lower layers in the deep hole structure layer.

[0039] Examples 15-16, Comparative Examples 6-7 Benzoic acid, isooctyl alcohol, and 5-aminotetrazole were added to phosphoric acid sequentially and heated and stirred to dissolve. Then, methyltrimethoxysilane was added at 80°C and reacted fully with phosphoric acid to obtain a stable polycrystalline silicon etching solution. The proportions of each substance are shown in Table 4. Table 4. Dosage and Proportioning of Polycrystalline Silicon Etching Solution

[0040] Example 17 Etching Experiment Etching experiments were conducted using the polycrystalline silicon etching solutions prepared in Examples 1, 15-16, and Comparative Examples 6-7. The etching and detection methods were the same as in Example 14. The results are shown in Table 5. Table 5. Etching effect of polycrystalline silicon etchant with different ratios

[0041] As shown in Table 5, in Example 15, the content of each additive was at the lower limit, and the etching rate and roughness still met the requirements. However, in Comparative Example 6, the additive content was too low, which resulted in the inability to form an effective etching inhibitor on the polycrystalline silicon surface. The generated bubbles could not quickly detach from the polycrystalline silicon surface, accompanied by silica back adhesion, which led to a deterioration in roughness and an etching difference of more than 10% between the upper and lower layers. In Example 16, the content of each additive was at the upper limit, and the etching rate and roughness still met the requirements. However, in Comparative Example 7, the additive content was too high, the water content was too low, the viscosity of the etching solution was too high, and the additives dehydrated and deteriorated under the action of phosphoric acid. Some of them failed and deposited on the polycrystalline silicon surface, resulting in a deterioration in roughness and a huge difference in etching amount between the upper and lower layers.

Claims

1. A polycrystalline silicon etching solution, characterized in that: The raw materials include the following percentages by mass: 1.0-1.5% aromatic carboxylic acids, 1.5-2.0% silane coupling agents, 0.5-1.0% tetrazolium compounds, 1.2-1.8% fatty alcohol compounds, 90.0% phosphoric acid, and the balance being deionized water. The aromatic carboxylic acid is any one of benzoic acid, salicylic acid, p-hydroxybenzoic acid, o-methylbenzoic acid, 3,5-dimethylbenzoic acid, p-aminobenzoic acid, and m-aminobenzoic acid; the silane coupling agent is one or more of methoxytrimethylsilane, ethoxytrimethylsilane, methyltrimethoxysilane, methyltriethoxysilane, ethyltriethoxysilane, aminopropyltrimethoxysilane, aminopropyltriethoxysilane, and γ-aminoethylaminopropyltrimethoxysilane; the tetrazolium compound is any one of tetrazolium, 1-methyl-1H-tetrazole, 5-methyltetrazole, 5-aminotetrazole, and 5-amino-1-methyltetrazole; the fatty alcohol compound is any one of n-heptanol, 2-heptanol, 3-heptanol, 4-heptanol, cycloheptanol, n-octanol, 2-octanol, 3-octanol, 4-octanol, isooctanol, cyclooctanol, and cyclooctanol.

2. The application of the polysilicon etching solution as described in claim 1 in deep-hole polysilicon etching.

3. The application according to claim 2, characterized in that: The aspect ratio of the deep-hole polycrystalline silicon is 200-300.

4. The application according to claim 2, characterized in that: The etching temperature is 165-175℃.

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